$\sum\limits_{n=1}^{3 }{{(n-1)}}=$
What is the question asking for? $\sum$ tells us to find the sum. The question is asking for the sum of the values of $n-1$ from $n = 1$ to $n = 3$. Evaluating $\begin{aligned} \sum\limits_{n=1}^{3 }{({n-1})}&= (1-1) + (2-1) + (3-1) \\\\ &= 0 + 1 + 2 \\\\ &= 3\end{aligned}$ The answer $\sum\limits_{n=1}^{3 }{({n-1})}=3$